The measurement of the total concentration of all forms of CO.sub.2 in fluid samples, i.e. dissolved gas, carbonic acid, bicarbonate ions, and carbonate ions, is generally difficult to accomplish with methods currently known and in use. All these forms exist in chemical equilibrium in aqueous solutions, and methods of measurement depend either on conversion of all forms to a single one that can be measured, or upon measurement of two or more parameters of the equilibrium system combined with calculation of the rest from known equilibrium constants. The most common method in use today consists of measuring the pH of the solution, and the partial pressure of dissolved CO.sub.2 gas with a Severinghaus-type electrode, combined with various calculation routines or graphical nomograms used to estimate the remaining parameters. This method has a number of problems: the measurement of pH is a logarithmic one, so small errors may cause relatively large errors in the calculation of other parameters; the CO.sub.2 electrodes are subject to drift, have a very slow response at very low partial pressures, and have an unacceptable loss of sensitivity and response time at temperatures much below human body temperature.
A second method no longer used commonly depended on conversion of all forms of CO.sub.2 to CO.sub.2 gas by acidification, followed by measurement of the gas evolved in a manometric apparatus. This method, too, is subject to numerous errors, principally volumetric errors in measurement of the several reagents employed, temperature fluctuations, and mercury manometer manipulation errors. In addition, the method is tedious and time-consuming, requires considerable skill, and is little used today.
A third method developed by this petitioner (not patented) depends upon acidification of the fluid sample to convert all combined forms of CO.sub.2 to dissolved CO.sub.2 gas, as in the method just outlined above, but the acidification is carried out in a small closed chamber containing a CO.sub.2 electrode. The increase in partial pressure in this small volume upon acidification of an unknown sample is compared with the partial pressure increase resulting from acidification of a known standard in order to calculate the concentration of the unknown. This method suffers many of the same problems inherent in the others, especially the inherent response and drift characteristics of the CO.sub.2 electrodes.
Conductivity change in alkaline solutions upon absorption of CO.sub.2 has been employed to measure the CO.sub.2 in gas samples, but the principal has not been applied in a direct way to small fluid samples, and the temperature dependence of the conductivity change has not been dealt with effectively. There is clearly a need for a new method, or improvements in present methods for making a simple, direct measurement of the total of all forms of CO.sub.2 in samples of liquids.